The aims of this proposal are to determine the distribution of myocardial current density during defibrillation and its effect on defibrillation success. One hypothesis of this study is that a minimum current density must be reached within the myocardium in order to achieve defibrillation. A second hypothesis is that this threshold current density is invariant with respect to a given species under homeostatic conditions. Whether threshold myocardial current density is achieved for a given defibrillation attempt depends on the distribution of current throughout the myocardium. This distribution is complex due to inhomogeneity of the heart volume conductor and anisotropy of myocardial conductivity and, in addition, may be altered by such factors as electrode size and placement as well as the presence of acute ischemia and infarcted tissue. A systematic study will be performed to determine the quantitative relationships between these parameters and the myocardial current distribution. The long-term objective is to optimize the delivery of the defibrillation pulse for a given set of clinical conditions. Due to limitations of both experimental and numerical approaches to this problem, this investigation will combine both methods in a complementary fashion. A numerical torso model will be developed for finite element analysis. This model will ultimately be able to predict the current distribution throughout the heart for a given set of transthoracic or transcardiac conditions. In parallel with this work will be the development of a 256 channel data acquisition system for mapping three-component epicardial current density, one-component current density within the myocardium, and activation wavefronts before and after the defibrillatory pulse. The numerical model will be checked against experimental data and modified until the numerical results sufficiently match the experimental data. The two methods will be combined to test the hypotheses and to determine the current density threshold. The model will then be used to perform numerical experiments for any set of defibrillation conditions.